Syphilis is the only bacterial STI for which proof of concept for vaccine development has been achieved, yet the complete protection reported by Dr. James N. Miller in 1973 was achieved only by 60 intravenous doses of motile ?-irradiated Treponema pallidum over 37 weeks. Although obviously impractical, this study emphasized the importance of surface antigens of T. pallidum and their native conformation. Our proposal embraces those critical concepts and unites the independent vaccine-related studies of two laboratory groups, providing a two- pronged approach to development of an effective syphilis vaccine that will (1) prevent development of the infectious ulcerative lesions to abolish transmission of T. pallidum and (2) inhibit treponemal dissemination through the host to prevent systemic disease and sequelae. Importantly, this proposal includes the evaluation of two human-track adjuvants designed to induce the Th1 immune responses required for protection. Through our respective basic research programs, we have identified two separate groups of highly conserved treponemal peptides that, when used as immunogens, inhibit (a) chancre development or ulceration (the N-terminal portions of TprK and Tpr subfamily I) and (b) treponemal dissemination in the host (Tp0751 [pallilysin]). This proposal builds on a two decade history of work to define protective antigens of T. pallidum, including optimized methods for recombinant antigen production (soluble pallilysin and refolded TprK and Tpr subfamily I); structural analyses to establish that recombinant proteins are in a close-to-native conformation; genomics-based knowledge of epitopes conserved among strains of T. pallidum; careful adjuvant selection to ensure use of a human-approved formulation that generates the required humoral and cellular immunity; and identification of possible correlates of protection. To accomplish our long-term goal of developing an effective vaccine for syphilis, we propose to (1) determine the combined immunoprotective capacity of the tri-antigen vaccine candidate cocktail in rabbits, using carefully refolded recombinant conserved N-terminal portions of TprK plus Tpr Subfamily I and soluble recombinant pallilysin, with two optimized human-track custom adjuvants; (2) determine the correlates of protection against syphilis, including cytokine production and multiple antibody functions; (3) determine the duration of immunity afforded by the vaccine cocktail prepared in the optimal custom adjuvant; and (4) determine the capacity of the tri-antigen cocktail to protect against infection by four additional strains of T. pallidum. Th successful accomplishment of these aims will lead the way toward development of an effective and broadly protective vaccine to prevent an infection that currently affects nearly 11 million adults, children, and infants each year.